Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!

Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.

I have an 06 STI that I bought modded from the previous owner. It is currently tuned to 21 psi and uses a MBC for boost control. My question is, is it safe to turn the boost down to say about 18 psi by adjusting the MBC, or would I also need to get the car tuned again since the current tune is set to 21 psi?

Depends on who tuned it.. In some cases tuners only do it for a small variance of 1-2 psi.... unless they tuned it for stages like 10,15,21 psi then you could hit AFR's that arent desirable as well as too much/too little timing.

Depends on who tuned it.. In some cases tuners only do it for a small variance of 1-2 psi.... unless they tuned it for stages like 10,15,21 psi then you could hit AFR's that arent desirable as well as too much/too little timing.

This

Some tuners just set the car to target boost and get the car running correctly there, while the other parts of the table haven't been touched. Lowering the boost could throw you into an untuned region of your map, if so then anything could happen...too much timing, too little timing, too much fuel, too little fuel, etc.

If the car was tuned properly for all conditions up to and including the final target boost, then you'd be fine, but there's no guarantee that it was.

Some tuners just set the car to target boost and get the car running correctly there, while the other parts of the table haven't been touched. Lowering the boost could throw you into an untuned region of your map, if so then anything could happen...too much timing, too little timing, too much fuel, too little fuel, etc.

If the car was tuned properly for all conditions up to and including the final target boost, then you'd be fine, but there's no guarantee that it was.

You guys are correct. Hopefully the tuner did tune the timing map but you wouldn't know until you looked at the map.

Quarter throttle and full throttle with less boost are different stories though :-) whats fun is tuning the newer STI's and setting up different boost maps for I, S and S# then having to tune it all those times. But when its done... holy crap does it make it fun :-)

If you turn down the boost say 2psi or 3psi to run it "safer" you might go from hitting the normal 3.5 down to a peak of 3 now. at half throttle you may only see the 2 load range... i think they are different in tuning... when i think of 1/4 thorttle i think of just barely in boost... my setup may be different then others but... that's all i ment. We are all agreeing here anyways. :-)

Thank you guys for all the great information. I decided that I will just leave it alone for now use my foot to control the boost. Although, I will slightly be adjusting it lower since its getting colder, thus causing the boost to creep up slightly.

I think what's being discussed here is whats the difference between lets say 2.5 load at full throttle with low boost, or 2.5 load high boost at partial throttle.

Exactly.

Obviously if the load is different, that matters. That's not the point.

The point is that the very same boost levels (and the load cells) that you get at part-throttle can be achieved at WOT if you play with WGDC or an MBC. And I don't see why you'd need to retune for the difference.

If I remember right, WOT pulls at 30% throttle give me exactly the same boost as WOT pulls at 100% throttle and 0% WGDC. I actually did some experimentation to figure that out, because I figure it doesn't matter what the WGDC is boost levels lower than whatever you get with 0% WGDC, or at throttle levels lower that whatever it takes to make that much boost.

I got curious one day, and revised my DBW tables to cap them at progressively lower throttle openings until I got down to WG boost levels, and 30% was it. So with my foot to the floor, the throttle plate went to 30%, and I got 10 or 11 psi, which is the same boost I get with WOT (really WOT this time) and 0% WGDC.

That's with an ATP 3076 and the IWG and actuator that it came with. I'm sure it would vary with different turbos and WGs and WG springs.